Ferromagnetic Negative Charge-Transfer Insulator: from Theoretical Proposal to Material Realization

TL;DR

This paper introduces ferromagnetic negative charge-transfer insulators (FNCTI), proposing their unique magnetic interactions and demonstrating a specific material, CrAs monolayer, as a realization with high Curie temperature and strong ferromagnetic exchange.

Contribution

It presents the first theoretical proposal and material realization of FNCTI, highlighting their distinct magnetic properties and validating long-standing hypotheses.

Findings

CrAs monolayer identified as FNCTI with a 0.35 eV band gap

Strong ferromagnetic exchange coupling (~57/40 meV)

High Curie temperature (~1500 K)

Abstract

Here we propose another type of ferromagnetic semiconductors: ferromagnetic negative charge-transfer insulator (FNCTI). In FNCTI, the negative charge-transfer states strongly enhance the ferromagnetic (FM) exchange interactions and the orbital hybridization gap permits the magnetic molecular orbitals as the underlying magnetic units rather than local atomic orbitals. Thus the FM exchange interactions are rather strong and decay slowly due to the large spearding of magnetic molecular orbitals. This is distinct from the superexchange mechanism where FM exchange interactions are quite weak as summarized in the well-known Goodenough-Kanamori-Anderson semi-empirical rules.Through first-principle calculations with the hybrid functional, PbO-type CrAs monolayer is mapped out to be a FNCTI, which possesses a band gap $\sim$ 0.35 eV, FM nearest-/next-nearest-neighbor exchange coupling strength $\sim$ 57/40 meV, and a high $T_c$ $\sim$ 1500 K respectively. It is believed that the existence of FNCTI validates the long-pending hypothesis by D. I. Khomskii and G. A. Sawatzky in 1997 [Solid State Commun. 102, 87 (1997)].